1. Field of the Invention
The present invention relates to processes for the removal of volatile hydrocarbon monomers from polymerization wastewater streams, and more particularly relates to the removal of acrylonitrile from polymerization wastewater streams.
2. Description of the Related Art
Acrylonitrile is the basis for a family of important commercial polymers. For example, copolymers of butadiene with less than 50% acrylonitrile provide oil resistant polymers. Copolymers of alkyl acrylates with acrylonitrile provide a range of polymer products. For example, when ethyl acrylate is the major component, oil elastomers are obtained; and when acrylonitrile is the predominant comonomer, useful molded plastic materials are obtained. Copolymers of a major proportion of styrene and minor proportions of acrylonitrile are also useful in modifying vinyl chloride polymers, as moldable plastics and as components as ABS resin blends. Normally, these acrylonitrile containing polymers are prepared by polymerization in water. Even when such polymerizations are substantially complete, there still remains an undesirable amount of unpolymerized acrylonitrile dissolved in the water on the order of several thousand to several hundred parts per million. Such wastewater streams typically result from water intensive polymerization processes such as emulsion polymerization, suspension polymerization and bulk/suspension polymerization.
Before such wastewater streams can be discharged into the environment, they must be properly processed to remove any solid polymerization products such as latex and resin; the pH of the wastewater stream must often be adjusted from pHs substantially different from 7, for example pHs below 3 or pHs above 10, to a pH of approximately 7.0; the wastewater stream then must have suspended solids removed by primary clarification which involves the addition of flocculent aids to form sludge which is then thickened and dewatered for landfill disposal, the effluent from the primary clarifier is then often cooled by methods such as spray coolers to reduce the temperature of the wastewater stream from a high temperature such as 120.degree. F., to a lower temperature such as 105.degree. F. to permit the activity of mesophilic organisms during an aeration step. During the aeration step, the mesophilic organisms reduce the biological oxygen demand of the water by consuming dissolved organic materials that are held therein by the introduction of waste activated sludge and air into an aeration basin. The product from the aeration process is then taken to a secondary clarifier where the water effluent therefrom can be further treated and filtered prior to environmental discharge within the regulatory parameters. A problem with such a process can be that volatile hydrocarbon monomers, for example acrylonitrile monomers, in the original wastewater stream, can volatilize prior to treatment in the aeration basin, for example the monomers can volatilize in the neutralization basins of the neutralization process, from the primary clarifier, or most likely from the spray cooler process.
Accordingly, there is a need to reduce the residual volatile hydrocarbon monomers, for example acrylonitrile, from the wastewater stream, prior to aeration of the wastewater stream, and more particularly prior to spray cooling of the wastewater stream and/or primary clarification of the wastewater stream.